Cathode for a Cell of a Lithium-Ion Battery, Its Manufacturing Process and the Battery Incorporating It

ABSTRACT

The invention relates to a cathode that is usable in a cell of a lithium-ion battery comprising an electrolyte based on a lithium salt and on a non-aqueous solvent, to a process for manufacturing this cathode and to a lithium-ion battery having one or more cells incorporating this cathode. This cathode is based on a polymer composition, obtained by melt processing and without solvent evaporation, that is the product of a hot compounding reaction between an active material and additives including a polymer binder and an electrically conductive filler. According to the invention, the binder is based on at least one crosslinked elastomer and the additives furthermore comprise at least one non-volatile organic compound usable in the electrolyte solvent, the composition advantageously includes the active material in a mass fraction greater than or equal to 90%.

FIELD

The present invention relates to a cathode that is usable in a cell of alithium-ion battery, to a process for manufacturing this cathode, and toa lithium-ion battery having one or more cells incorporating thiscathode.

BACKGROUND

There are two main types of lithium storage battery: lithium-metalbatteries, where the negative electrode is made of lithium metal (whichmaterial causes problems with safety when in the presence of a liquidelectrolyte), and lithium-ion batteries, where the lithium remains inthe ionic state. Lithium-ion batteries consist of at least twoconductive Coulombic electrodes of different polarities, the negativeelectrode or anode (generally made of graphite) and the positiveelectrode or cathode (generally made of an oxide of a transition metal,such as an oxide of vanadium or cobalt, or made of a lithiated ironphosphate such as, for example, described in documents U.S. Pat. No.6,514,640 or WO-A1-2011/092283), between which electrodes a separator islocated, which separator consists of an electrical insulator imbibedwith an aprotic electrolyte based on Li⁺ locations ensuring the ionicconductivity. The electrolytes used in these lithium-ion batteriesusually consist of a lithium salt, for example of formula LiPF₆, LiAsF₆,LiCF₃SO₃ or LiCO₄, which is dissolved in a mixture of non-aqueoussolvents such as acetonitrile, tetrahydrofuran, or more often acarbonate, for example of ethylene or propylene.

The active material of the cathode of a lithium-ion battery allowsreversible insertion/removal of lithium into/from this cathode, and thehigher the mass fraction of this active material in the cathode, thehigher its capacity. The cathode must also contain an electricallyconductive compound, such as carbon black and, in order to provide itwith sufficient mechanical cohesion, a polymer binder. A lithium-ionbattery is thus based on the reversible exchange of lithium ions betweenthe anode and the cathode during the charging and discharging of thebattery, and, for a very low weight, by virtue of the physicalproperties of lithium, such a battery has a high energy density.

The cathodes of lithium-ion batteries are most often manufactured usinga process comprising, in succession, a step of dissolving or dispersingthe various ingredients of the cathode in a solvent, a step of spreadingthe obtained solution or dispersion on a metallic current collector, andthen lastly a step of evaporating this solvent. Many types of polymerbinders can be used, among which mention may firstly be made of PVDF(polyvinylidene fluoride), which is more easily compatible with acathode operating at a high operating voltage (more than 4 V) because ofthe presence of fluorine, but also, for example, polyacrylonitriles(PAN) with polybutylacrylate latexes.

Processes for manufacturing the cathodes of lithium-ion batteries thatuse an organic solvent have many drawbacks with respect to theenvironment and safety. In particular, it is, in this case, necessary toevaporate large amounts of such solvents, which are toxic orinflammable.

As for processes that use an aqueous solvent to manufacture thesecathodes, their major drawback is that the cathode must be verythoroughly dried before they can be used, traces of water being known tolimit the useful lifetime of lithium storage batteries.

It is therefore highly desirable, for lithium-ion batteries, to preparecathodes that are manufactured without using solvents. It is in thiscontext that processes for manufacturing cathodes for lithium-ionbatteries using melt processing techniques (for example extrusion) havebeen described in the literature.

Unfortunately, these melt processes cause major difficulties in the caseof lithium-ion batteries, which, as is known, require a mass fraction ofactive material in the polymer mixture of the cathode of at least 90%for the latter to have sufficient capacity in the lithium-ion battery.However, at such contents of active material the viscosity of thecathode polymer mixture becomes very high, and leads to a risk of themixture over-heating or losing its mechanical cohesion once it is inuse.

Document U.S. Pat. No. 6,939,383 describes the extrusion of a polymercomposition comprising a poly(ethylene oxide)-poly(propyleneoxide)-poly(glycidyl ether) copolymer for a ionically conducting polymerfor solventless implementation of a cathode for a lithium-polymerbattery. However, the mass fraction of active material in the singlecathode polymer composition manufactured in this document is only 64.5%.

Document U.S. Pat. No. 5,749,927 discloses a process for the continuouspreparation of lithium-polymer batteries by extrusion, which processcomprises compounding the active material with an electrical conductorand a solid electrolyte composition comprising a polymer, a lithium saltand a propylene carbonate/ethylene carbonate mixture greatly in excessover this polymer. In this document, the mass fraction of activematerial present in the cathode polymer composition is also below 70%.

Thus, a major drawback of these known melt processes for manufacturingcathodes for lithium storage batteries is that the mass fractions ofactive material in the cathode polymer composition are insufficient toobtain high-performance cathodes specifically for lithium-ion batteries.

SUMMARY

One aim of the present invention is therefore to provide a process, formanufacturing a cathode, that overcomes all of the aforementioneddrawbacks, and this aim is achieved since the Applicant has discovered,surprisingly, that if an active material and additives comprising acrosslinked elastomer matrix, an electrically conductive filler and anon-volatile (i.e. having a boiling point above 150° C. at anatmospheric pressure of 1.013×10⁵ Pa) organic compound, are hotcompounded, without solvent evaporation, then a cathode polymercomposition is obtained that is usable in a lithium-ion batterycomprising an electrolyte based on a lithium salt and on a non-aqueoussolvent, with a fraction of this active material in the compositionclearly higher than those obtained previously by melt processing, andadvantageously greater than or equal to 90%, the one or more organiccompounds advantageously being used as a solvent for the electrolyte.

DETAILED DESCRIPTION

A cathode according to the invention, usable in a cell of a lithium-ionbattery comprising an electrolyte based on a lithium salt and on anon-aqueous solvent, is thus based on a polymer composition, obtained bymelt processing and without solvent evaporation, that is the product ofa hot compounding reaction between an active material and additivescomprising a polymer binder and an electrically conductive filler, andthe cathode is such that the binder is based on at least one crosslinkedelastomer and such that these additives furthermore comprise at leastone non-volatile organic compound usable in this electrolyte solvent,the composition comprising the active material in a mass fractionadvantageously greater than or equal to 90%.

It will be noted that this very high mass fraction of active material inthe cathode according to the invention ensures that the or each cellobtained is a high-performance cell and therefore that the lithium-ionbattery incorporating them is a high-performance battery.

It will also be noted that the uniform distribution of said at least onecrosslinked elastomer in the composition ensures the mechanical strengthof the cathode.

Advantageously, said active material may comprise at least one lithiatedpolyanionic compound or complex having an operating voltage below 4 Vand preferably carbon coated, such as a lithiated metal M phosphate offormula LiMPO₄ (also called phosph-olivine), such as a carbon-coatedlithium-iron phosphate having the formula C—LiFePO₄.

Specifically, it will be noted that the active material used in thecomposition of the present invention may consist of elementarycarbon-coated particles or agglomerates of elementary particlescomprising a carbon coating or deposit.

Preferably, said at least one elastomer is a peroxide-crosslinked dieneelastomer and, even more preferably, a hydrogenated nitrile rubber(HNBR). Also preferably, said at least one elastomer may be present insaid composition in a mass fraction between 1% and 5%.

Advantageously, said at least one non-volatile organic compound maycomprise a carbonate, preferably a carbonate of at least one olefin suchas ethylene, which is preferably used in the electrolyte composition.

It will be noted that using such a carbonate, such as an ethylenecarbonate, advantageously allows:

-   -   the filler content in the composition to be increased;    -   the inherent risks with respect to the toxicity of volatile        organic compounds (VOC), used in conventional processes for        manufacturing cathodes, to be avoided because this carbonate is        a product that is solid at room temperature and much less        hazardous to handle; and    -   the cathode polymer composition to be used without evaporating        the carbonate beforehand, and the incorporation of the        electrolyte into the cathode to be made easier because this        carbonate is one of the main constituents of electrolytes used        at the present time in lithium-ion batteries.

Also advantageously, said at least one organic compound may be presentin said composition in a mass fraction between 0.1% and 5%.

It will be noted that the invention allows the salts required foroperation of the cathode to be incorporated during its manufacture.

According to another feature of the invention, said additives mayfurthermore comprise a crosslinking system that is present in thecomposition in a mass fraction between 0.05% and 0.20%, and thatpreferably comprises an organic peroxide and a crosslinking co-agent inthe case where said at least one elastomer is a diene elastomer such asa hydrogenated nitrile rubber (HNBR).

According to another feature of the invention, said electricallyconductive filler may be chosen from the group consisting of carbonblack, graphite, expanded graphite, carbon fibres, carbon nanotubes,graphene and their mixtures, and is present in the composition in a massfraction between 1% and 5%.

A process, according to the invention, for manufacturing a cathode suchas defined above, is characterized in that it comprises:

-   -   a) melt compounding, without solvent evaporation, in an internal        mixer or an extruder, of said active material and said additives        comprising said binder and said organic compound in the solid        state in order to obtain said composition in the crosslinkable        state, this active material preferably comprising at least one        lithiated polyanionic compound or complex such as a        carbon-coated lithiated iron phosphate of formula C—LiFePO₄; and    -   b) crosslinking and optionally hot forming this composition, in        order to obtain said crosslinked composition.

According to another feature of the invention, step a) may be carriedout by compounding said binder into a powder premix of the otheringredients of the composition, for example at a temperature between 60°C. and 80° C. in an internal mixer.

According to another feature of the invention, step b) may be carriedout by hot pressing the crosslinkable composition.

Advantageously, the process of the invention may then comprise a step c)of rolling said crosslinked composition so as to deposit it on ametallic current collector with which said cathode is equipped.

A lithium-ion battery according to the invention comprises at least onecell comprising an anode, for example a graphite-based anode, a cathodesuch as defined above, and an electrolyte based on a lithium salt and ona non-aqueous solvent.

According to another advantageous feature of the invention saidelectrolyte solvent may comprise said at least one non-volatile organiccompound of the cathode.

According to another aspect of the invention, said cathode comprises ametallic current collector making contact with at least one film made ofsaid polymer composition.

Other features, advantages and details of the present invention willbecome clear from reading the following non-limiting description of anexemplary embodiment of the invention, given by way of illustration.

Example 1

A cathode polymer composition was prepared in an Haacke internal mixerat 70° C., the composition comprising the following constituentsexpressed in mass fraction (%):

HNBR binder (“Therban 4307”) 2.68 Carbon black 2.68 Ethylene carbonate0.54 Active material C—LiFePO₄ 93.97 Crosslinking system: Dicumylperoxide 0.08 Triallyl cyanurate (TAC) 0.05

The various compounds were introduced into this internal mixer insuccession, starting with the hydrogenated nitrile rubber by way of acrosslinkable diene elastomer (HNBR binder), and then a premix in powderform of the other ingredients above. After this compounding, and hotpressing at 170° C. for 10 minutes simultaneously allowing the binder tobe crosslinked, a 1 mm-thick electrode able to form a cathode inside acell of a lithium-ion battery, after deposition on a current collectorwith which this cathode is equipped, was obtained directly.

It will be noted that the very high mass fraction (higher than 93%) ofactive material in this cathode ensures that the or each cell obtainedis a high-performance cell and therefore that the lithium-ion batteryincorporating them is a high-performance battery.

“Control” Example, not According to the Invention:

A “control” composition having the same formulation (i.e. the sameamounts of the same ingredients) as that of Example 1 was prepared bydispersing/dissolving said ingredients in a MIBK (methyl isobutylketone) solvent—i.e. using a process that did not conform to the meltprocess of the invention—this “control” composition being deposited bycoating on a current collector.

It has be observed that the “control” cathode obtained bydispersing/dissolving had intrinsic physical properties that were verydifferent to those of the cathode of Example 1, especially in terms ofmorphology (scanning electron microscope “SEM” micrographs), bulkdensity and electrical conductivity, as may be seen in Table 1 below.

In particular, it may be noted that the bulk density of the cathodeobtained by melt processing (Example 1, without solvent) is clearlyhigher than 1—lying between 1.5 and 2—i.e. more than two times higherthan the bulk density of the “control” cathode obtained with solvent.

TABLE 1 Cathode of “Control” Example 1 cathode Bulk density 1.854 0.777Electrical conductivity (S/cm) 0.0392 0.0065

1. Cathode usable in a cell of a lithium-ion battery comprising anelectrolyte based on a lithium salt and on a non-aqueous electrolytesolvent, the cathode being based on a polymer composition, obtained bymelt processing and without solvent evaporation, that is the product ofa hot compounding reaction between an active material and additivescomprising a polymer binder and an electrically conductive filler,wherein said binder is based on at least one crosslinked elastomer andin that said additives furthermore comprise at least one non-volatileorganic compound usable in said electrolyte solvent, the compositioncomprising said active material in a mass fraction greater than or equalto 90%.
 2. Cathode according to claim 1, wherein said active materialcomprises at least one lithiated polyanionic compound or complex havingan operating voltage below 4 V.
 3. Cathode according to claim 1, whereinsaid at least one elastomer is a peroxide-crosslinked diene elastomer,preferably a hydrogenated nitrile rubber (HNBR).
 4. Cathode according toclaim 1, wherein said at least one elastomer is present in saidcomposition in a mass fraction between 1% and 5%.
 5. Cathode accordingto claim 1, wherein said at least one organic compound comprises acarbonate.
 6. Cathode according to claim 1, wherein said at least oneorganic compound is present in said composition in a mass fractionbetween 0.1% and 5%.
 7. Cathode according to claim 1, wherein saidadditives furthermore comprise a crosslinking system that is present insaid composition in a mass fraction between 0.05% and 0.20%, and thatcomprises an organic peroxide and a crosslinking co-agent in the casewhere said at least one elastomer is a diene elastomer.
 8. Cathodeaccording to claim 1, wherein said electrically conductive filler ischosen from the group consisting of carbon black, graphite, expandedgraphite, carbon fibres, carbon nanotubes, graphene and their mixtures,and is present in said composition in a mass fraction between 1% and 5%.9. Process for manufacturing a cathode according to claim 1, wherein themethod comprises: a) melt compounding, without solvent evaporation, inan internal mixer or an extruder, of said active material and saidadditives comprising said binder and said organic compound in the solidstate in order to obtain said composition in the crosslinkable state,this active material preferably comprising at least one lithiatedpolyanionic compound or complex such as a carbon-coated lithiated ironphosphate of formula C—LiFePO₄; and b) crosslinking and optionally hotforming this composition, in order to obtain said crosslinkedcomposition.
 10. Manufacturing process according to claim 9, whereinstep a) is carried out by compounding said binder into a powder premixof the other ingredients of the composition at a temperature between 60°C. and 80° C. in an internal mixer.
 11. Manufacturing process accordingto claim 9, wherein step b) is carried out by hot pressing saidcrosslinkable composition.
 12. Manufacturing process according to claim9, wherein said method then comprises a step c) of rolling saidcrosslinked composition so as to deposit it on a metallic currentcollector with which said cathode is equipped.
 13. Lithium-ion batterycomprising at least one cell comprising an anode, a cathode and anelectrolyte based on a lithium salt and on a non-aqueous solvent,wherein said cathode is as defined in claim
 1. 14. Lithium-ion batteryaccording to claim 13, wherein said electrolyte solvent comprises saidat least one non-volatile organic compound of the cathode. 15.Lithium-ion battery according to claim 13, wherein said cathodecomprises a metallic current collector making contact with at least onefilm made of said polymer composition.
 16. Cathode according to claim 1,wherein said active material comprises at least one lithiatedpolyanionic compound or complex having an operating voltage below 4 Vand that is carbon coated.
 17. Cathode according to claim 1, whereinsaid lithiated polyanionic compound or complex comprises lithiated metalM phosphate of formula LiMPO₄, where M is an iron atom.
 18. Cathodeaccording to claim 1, wherein said at least one elastomer is ahydrogenated nitrile rubber (HNBR).
 19. Cathode according to claim 1,wherein said at least one organic compound comprises a carbonate of atleast one olefin such as ethylene.
 20. Lithium-ion battery comprising atleast one cell comprising a graphite-based anode, a cathode and anelectrolyte based on a lithium salt and on a non-aqueous solvent,wherein said cathode is as defined in claim 1.